Various catalysts for polymerization to obtain olefinic hydrocarbon polymers have been proposed to date. Among them, widely employed catalysts soluble in inert hydrocarbon solvents are Ziegler catalysts composed of a vanadium compound and an organoaluminum compound. Known vanadium compounds typically include halogen compounds, alkoxy compounds, and acetylacetonate complexes. Olefin polymers obtained by using general Ziegler catalysts, however, have broad distributions of molecular weight and composition and therefore have poor transparency and are tacky, which are unsuitable for use as films or resin modifiers. For use in these applications, olefin polymers should have narrow distributions of molecular weight and composition.
There have been made several proposals as to a solution polymerization technique for obtaining olefin polymers having narrow molecular weight and composition distributions by using a vanadium-containing catalyst. For example, JP-B-62-45244 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses a process comprising undergoing polymerization at a relatively low temperature by using an organic solvent-soluble vanadium compound and an organoaluminum compound. According to the process, however, most of the polymerization active species formed by the reaction between the vanadium compound and the organoaluminum compound become insoluble in a polymerization solvent, and polymerization activity achieved is lower than in processes wherein a vanadium compound and an organoaluminum compound are not brought into contact with each other.
JP-A-59-140209 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a process in which a trihalogenated vanadium compound which is suitable for polymerization of olefinic hydrocarbons and insoluble in organic solvents is solubilized by reacting with an oxygen-containing electron donor. According to this process, the reaction of the trihalogenated vanadium compound and an oxygen-containing electron donor should be conducted at a relatively high temperature of about 100.degree. C. and, moreover, equipment for carrying out this reaction is required in addition to equipment for polymerization reaction.
Some of halogen-containing compounds generally known as a polymerization activator make it feasible to produce olefin polymers having narrow distributions of molecular weight and composition. Such compounds include perchlorocrotonic acid esters and .alpha.,.alpha.-dichlorophenylacetic acid esters. However, decomposition products of these halogen-containing compounds remain in polymers obtained, causing coloration or an odor.
JP-A-61-4708 teaches use of a reaction product formed between an alcohol having only one hydroxyl group in the molecule thereof and a vanadium compound having a specific structure as a catalyst. However, since polymers obtained by using such a catalyst have a relatively broad distribution of molecular weight, the process disclosed is not suitable to achieve the purpose of obtaining a narrow molecular weight distribution.
It is known that an amorphous olefin copolymer having a very narrow molecular weight distribution can be obtained by using a reaction product formed between an .alpha.,.gamma.-diketone having two oxygen atoms in the molecule thereof and a vanadium compound as reported, e.g., in Makromol. Chem., Rapid Commun., Vol. 8, p. 285 (1987). However, there is an industrial problem in that achievement of a narrow molecular weight distribution by this technique requires temperature control at minus several tens of degree C.
Further, JP-A-59-100105 discloses a process for carrying out polymerization in a gaseous phase or in a slurry state by using a catalyst-on-carrier containing a reaction mixture of a dihydroxy hydrocarbon compound and a vanadium compound. Where this catalyst is applied to solution polymerization, the resulting polymer has a broad molecular weight distribution and, besides, the catalyst tends to form a precipitate which clogs an apparatus for catalyst feeding and adheres to the inner wall of a polymerization vessel, causing great inconvenience in industrial production.